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1. Field of the Invention
This invention relates to phosphate stabilizing compositions comprising (a) polyaspartic acid, and (b) an anionic carboxylic polymer. The compositions effectively stabilize phosphates by inhibiting the formation of calcium phosphate scale. The invention also relates to a process for inhibiting calcium phosphate scale in water treatment systems.
2. Description of the Related Art
It is known to treat cooling water and many industrial waters with inorganic and organic phosphorous-containing compounds to prevent anodic corrosion. Typically used for this purpose are water-soluble phosphates such as ammonium and/or alkali metal phosphates, where the phosphates can be ortho-, meta- or pyrophosphates, particularly orthophosphates. Polyphosphates can likewise be employed because they are hydrolyzed to orthophosphates in aqueous medium.
The problem with using phosphates in water treatment is that they tend to produce calcium phosphate scales, which adhere to the metal surfaces of boilers and metallic heat exchangers. The scale inhibits effective heat transfer, restricts the flow of the water, and promotes the development of underdeposit corrosion. Consequently, it is necessary to remove the scale by cleaning. Such cleaning is expensive because equipment must be shutdown, labor costs are incurred, and production is delayed. In view of these problems, preventing scale formation is preferred to scale removal.
It is well known that phosphates can be effectively stabilized with anionic carboxylic polymers alone, or even more effectively with the blends of these polymers and organic phosphonates. See for example U.S. Pat. No. 4,584,105. Although these polymers are effective against a variety of scales, they are not always entirely effective against calcium phosphate scales and are costly to use.
It is also known to use phosphonates for calcium carbonate scale control and/or to enhance corrosion inhibition. For scale inhibition, the dosage of the phosphonate is typically in the order of 0.5-5 ppm, while for corrosion inhibition, in the order of 5-8 ppm. However, the use of phosphonates for these purposes is not desirable from an environmental or economic standpoint.
U.S. Pat. No. 5,152,902 discloses that polyaspartic acids inhibit calcium phosphate crystallization. However, no data is given in the patent, and it is known that polyaspartic acid is very weak phosphate scale inhibitor, if it does so at all, and it is necessary to lower the pH to provide calcium phosphate stabilization. For all practical purposes, polyaspartic acid has nearly zero efficacy against calcium phosphate scales.
U.S. Pat. No. 5,523,023 relates to compositions comprising polyaspartic acid and phosphonobutane tricarboxylic acid, which are used for alkaline cleaners. U.S. Pat. 5,386,038 discloses a water-soluble mixture of phosphonated oligomers that inhibit scale formation and/or the corrosion of metal exposed to aqueous systems. WO 00/44677 teaches that certain blends of polyaspartic acid and certain water-soluble mixture of phosphonated oligomers effectively inhibit the formation of calcium carbonate scale and are also effective corrosion inhibitors.
All citations referred to under this description of the xe2x80x9cRelated Artxe2x80x9d and in the xe2x80x9cDetailed Description of the Inventionxe2x80x9d are expressly incorporated by reference.
This invention relates to phosphate stabilizing compositions comprising (a) polyaspartic acid, and (b) an anionic carboxylic polymer. The compositions effectively stabilize phosphates by inhibiting the formation of calcium phosphate scale. The invention also relates to a process stabilizing phosphates from forming calcium phosphate scale in water treatment systems. The compositions provide synergistic phosphate stabilization in cooling waters, which is preferably accomplished without the use of a phosphonate or phosphonate oligomer.
The compositions are synergistic because, although polyaspartic acids are not effective phosphate stabilizers, blending polyaspartic acid with known polymer phosphate inhibitors improves the performance of known phosphate stabilizers. This was surprising because polyaspartic acid alone does not have any significant phosphate stabilizing effect. The mixtures stabilize phosphates more than was expected in view of the phosphate inhibition activity of the individual components. Although carboxylic polymers alone provide some phosphate stabilization, they are not biodegradeable and are expensive to use. On the other hand, polyaspartic acid is biodegradeable, less expensive, and inhibits corrosion.
The process is particular useful for treating aqueous systems containing a phosphate where the pH of the aqueous system is from about 8.0 to about 9.3.
Although not necessary or preferred, a water-soluble phosphonated oligomer having the general formula can be added to the composition:
H[CHRCHR]nxe2x80x94PO3M2 
wherein at least one R group in each unit is a COOM, CH2OH, sulphono, or phosphono group and the other R group which may be the same as, or different from, the first R group, is hydrogen or a COOM, hydroxyl, phosphono sulphono, sulphato, C1-7 alkyl, C1-7 alkenyl group or a carboxylate, phosphono, sulphono, sulphato, and/or hydroxy substituted C1-7 alkyl or C1-7 alkenyl group, and each M is a cation such that the phosphonated oligomer is water soluble and n is 1 to 6, typically  greater than 1 and  less than 6. These water-soluble phosphonated oligomers are typically added to the composition to inhibit calcium carbonate scale formation and/or corrosion. However, the amount of water-soluble phosphonate oligomer used in the compositions of this invention is such that the weight ratio of water-soluble phosphonate oligomer to polyaspartic acid is  less than 1:1 or greater than 1:9.
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The detailed description and examples will illustrate specific embodiments of the invention will enable one skilled in the art to practice the invention, including the best mode. It is contemplated that many equivalent embodiments of the invention will be operable besides these specifically disclosed. All units are in the metric system and all percentages are percentages by weight unless otherwise specified.
Component (a) of the scale inhibitor composition is a water soluble polyaspartic acid. For purposes of this invention, the term xe2x80x9cpolyaspartic acidxe2x80x9d shall be construed to include salts and derivatives of polyaspartic acid. Polyaspartic acid, salts thereof, and derivatives of polyaspartic acid are well known and are described in U.S. Pat. No. 5,523,023 which is hereby incorporated by reference. Preferably used is polyaspartic acid having an average molecular weight, according to gel-permeation chromatographic analysis, from 500 to 10,000, preferably 1,000 to 5,000, most preferably 2,000 to 4,000. The polyaspartic acid is preferably used as a salt, in particular as a sodium salt or potassium salt. Whether polyaspartic acid is used in the form of an acid or a salt depends upon the pH of the aqueous system treated. Preferably, the salts of polyaspartic acid are sodium salts. Derivatives of polyaspartic acid, for example anhydrides of polyaspartic acid, which can convert to polyaspartic acid by hydrolysis under use conditions, also can be used.
Component (b) is an anionic carboxylic polymer or a salt thereof that stabilizes phosphates against precipitation in an aqueous system. For purposes of describing this invention, polymer shall be construed to mean any product formed by the polymerization of one or monomers, and includes homopolymers, copolymers, terpolymer, tetrapolymers, etc. The anionic carboxylic polymer preferably has some stabilizing effect against the formation of calcium phosphate scale when used alone. The anionic carboxylic polymer typically has an average molecular weight of 1,000 to 50,000 as determined by gel-permeation chromatographic analysis, preferably from 2,000 to 10,000. These polymers and their method of synthesis are well known in the art.
Examples of monomers that can provide the source for the carboxylic functionality for the anionic carboxylic polymer include acrylic acid, maleic acid, methacrylic acid, crotonic acid, isocrotonic acid, fumaric and itaconic acid.
Numerous co-monomers can be polymerized with the monomer that is the source of the carboxylic functionality. Examples such monomer included vinyl, allyl, acrylamide, (meth) acrylate esters or hydroxy esters e.g. hydroxypropyl esters, vinyl pyrrolidone, vinyl acetate, acrylonitrile, vinyl methyl ether, 2-acrylamido-2-methyl-propane sulphonic acid, vinyl or allyl sulphonic acid and styrene sulphonic acid. The molar ratio of carboxylic functional monomer to other monomer varies over wide ranges, e.g. from 99:1 to 1:99, but more typically from 95:5 to 25:75.
Examples of hydrolyzed or partially hydrolyzed acrylamides/acrylates are disclosed in U.S. Pat. No. 4,001,161. Preferably used as the hydrolyzed or partially hydrolyzed acrylamides/acrylates are low molecular weight soluble polymers having average molecular weight of 500-10,000, most preferably from 2000-6000. Example of commercially available water-soluble hydrolyzed or partially hydrolyzed acrylamide/acrylates polymers is Cyanamer P-70 from Cytec Industries sold as a 50% aqueous solution.
The sulfonated styrene/maleic anhydride copolymers are high molecular weight water-soluble polymers typically having average molecular weight from 1,000 to 70,000, preferably from 15,000 to 70,000. Examples of sulfonated styrene/maleic anhydride copolymers are shown in U.S. Pat. Nos. 4,255,259 and 4,306,991. Examples of commercially available water-soluble sulfonated styrene/maleic anhydride copolymers are Versa TL-4 sold as a 25% aqueous solution, and Versa TL-3 sold as 95% solids, available from Alco Chemical.
It is also possible to employ carboxylic acid polymers that contain a chain phosphorus atom, which forms part of an acid group, preferably phosphino polycarboxylic acids. For a description of such polymers, see, for example, U.S. Pat. No. 4,692,317 and U.S. Pat. No. 2,957,931. The molecular weight of such polymers is relatively low, generally below 6,000, the preferred molecular weight being from 500 to 6000. A particularly suitable polymer is that sold as Belclene 500, and DP-3385 sold as a 40% aqueous solution and Belsperse 161 sold as a 50% aqueous solution, which are available from Biolab.
The effective ratio of carboxylic polymer to polyaspartic acid is from about 1:9 to about 9:1, with the best synergistic efficacy from 1:4 to 2:1. The compositions are effective at a pH range of 7.0 to 9.2, preferably at a pH range of 8.0 to 8.9, and most preferably at a pH range of 8.2 to 8.6 at temperatures of 5xc2x0 C. to 98xc2x0 C. The phosphate stabilizing compositions are used at the minimum dosage of 0.1 ppm to the maximum of 500.0 ppm, but preferably 1.0 ppm to 20.0 ppm actives.
Other optional components include phosphonobutane tricarboxylic acid, tolyltriazole, orthophosphate, polyphosphates, phosphates, hydroxyethylidene diphosphonic acid, amino tri (methylene phosphonic acid).